Energy & Fuels 1992,6,374-386
374
(10 pm is not correct. Nevertheless, the data do indicate the range of particle sizes that concerned us in this work. These distributions were also used for the calculations of expected purity. General Separation Procedure. For all the CFC separations, the coal slurry, at the appropriate density, was fed into the rotor and then followed by -250 mL of flushing solution to clear the rotor of coal. Aqueous CsCl with 8 g/L Brij-35 surfactant was used for all the separations. Except where noted, the actual fractionation of the coals was done in one of two ways. For those separations starting at a high density, with successive fractions or cuts at lower densities, the floating coal material was never isolated. Aliquota of the float phase were taken to determine the weight and purity of the material. Only the sink phases were filtered, washed, and dried. Subsequent separations were done by adjusting the density of the float slurry to the desired new density and passing this slurry through the centrifuge. For those separations starting at a low density, both the float and sink were isolated, washed, and dried. The sink material was then resuspended in a CsCl/Brij-35 solution of the appropriate density. ~
(5) Lloyd, W. C.; Riley, J. T.;Kuehn, K. W.; Kueh?,D. W. 'Chemistry and Reactivity of Micronized Coals", Final Technical Report DOE/ PC/80514, 1988.
1.0
e
-
0.8-
Particle Size ( y M )
Figure 5. Relative volume distribution w particle size, obtained by laser scattering.
In our earlier work, three phases were usually collected: A float phase, which was the rotor effluent, the true sink phase, which was the material found deposited on the rotor wall, and the rotor slurry, which was the material that was discharged when the rotor stopped spinning. This rotor slurry usually contained more float material than the rotor wall deposit, and, hence, was less pure. However, it also contained from 40 to 60% of the entire sink material. Because of this large amount of sink,we did not feel that the rotor slurry material should be neglected. Thus, the sink data that is displayed in the present work is the combination of both the rotor slurry and rotor wall deposit fractions. The separated phases were monitored by ADGC and the reported purities were derived from integration of the density distribution patterns, as in part 1. For instance, float purity for our purposes is the fraction of material found by integrating the mass normalized density distribution curve from the lowest density of the curve to the solution (or cut) density. Separation of APCS 7. This coal was previously used to establish the working conditions of CFC for maceral ~, separation.' Originally, two densities, 1.24 g ~ m -which mimicked the separation of liptinite from vitrinite and inertinite, and 1.335 ~ m -which ~ , mimicked the separation of inertinite from liptinite and vitrinite, were used for these
Coal Maceral Separation. 2
Energy & Fuels, Vol. 6, No. 4, 1992 377
11.102F
Figure 6. Multiple density fractionation on APCS 7. The dashed curve represents the sink fraction. The solid curve represents the float fraction and the vertical line represents the cut density. Starting feed for low-density cut at 1.192 g cm-3 = 233 g. Starting feed for high-density cuts = 150 g. For both cases: [Coal] = 100 g/L; flow rate = 25 mL/min; centrifuge speed = -45000 rpm.
tests. For the present study, the sink fractions from the liptinite separation of the original work were combined, and then used for the high-density separation of inertinite materials. Correspondingly,the float fractions from the former inertinite separations were combined and then used for the low-density cuts to isolate liptinite. The course of the further separations of these materials is displayed in Figure 6 and Table III. In Figure 6,successive horizontal plots in a float or sink fractionation series represent a recycle of the initial product at the same density. The plots in vertical succession represent further density fractionation at lower or higher densities. Vertical lines represent the density used in a particular cut. Each set of curves show the float and sink phase density gradient patterns we observed. Unfortunately, the large disparity in amounb of material separated at different densities does not permit us to represent each data curve as the actual masses being separaM, some of the data would be almost invisible. Thus, all the plots have been normalized to the highest value. The starting coal concentrationwas 100 g/L for both separations. The lower density fractionation was carried out on the material that had originally been fractionated at a density ~ , amounted to 233 g of coal. Only one of 1.335 g ~ m -and . the further density cut was made: 1.192 g ~ m - ~However, float fraction was recycled twice at this density. A density of 1.192 g cm-3 is low for a liptinite cut, but we needed quite pure liptinite material for other work. Thus, we picked a density low enough to ensure that the amount of contaminating vitrinite would be small (see Discussion section). The first cycle was, in fact, a very poor separation. The enrichment of float material increased from the original sample at 7.0-16.9%. The reason for such poor behavior is that the rotor was severely overloaded. Table IV shows the purity and also the quantity of material that would be predicted at various densities, if no separation of the original coal occurred. The desired float material at 1.192 g cm-3 represents only 5.3% of the total coal sample. Thus, we expect 14 times as much sink material. (The section in part 1 on rotor overloading explores the actual separation data in detail.) About 200 g of rotor deposit would then be expected to deposit'in the rotor. However, the Model 'K" clarifier rotor can hold only !50-75
Table IV. Percent Purity of Float and Sink Fractions Expected for APCS 7 at Various Densities (If There Was No Separation) density, g cm-3 float, wt % sink, wt 7% 1.192 5.3 94.7 11.0 89.0 1.236 1.265 35.1 64.9 1.310 87.4 12.6 1.335 91.9 8.1 1.365 96.3 3.6
g of material before significant degradation of the purity of the float fraction will occur. When the float material from this overloaded separation was recycled, we obtained a very good separation (90% pure float phase). This material was then recycled again to see if further improvement was possible. The final result was a float fraction that was 99.2% pure. The high density separation series for APCS 7 startad with 150 g of combined sink material from our previous The . data test separations run at a density of 1.24 g ~ m - ~ are also shown in Figure 6 and Table 111. After the first ~ ,isolated a sink product that separation at 1.365 g ~ m -we was 83.8% pure and a float product that was 97.0% pure. We recycled this float, to see if more sink material could be extracted from the float fraction. (There was still approximately 4.5 g of sink material in the fraction.) The recycle step produced a new float purity of 95%. This value is lower than the original, but the errors in our ADGC analysis procedures are about i5%. At such high-purity values, the ADGC and subsequent analysis procedures are obviously approaching the limits of applicability. The recycled sink fraction we obtained was only 64.2% pure. Nevertheless, this purity still represents a very high separation efficiency, since there was only -3% of this material in the original float sample. No recycle of the original CFC sink material was done, although we are certain that ita purity could have been improved by recycling. We next subjected the 1.365g cm-3 recycled float phase to a single-cycle separation at a density of 1.310 g ~ m - ~ . The float phase resulting from this separation was then further processed at a density of 1.265 g cm+. All the primary separation cycles produced sink phases that were >70% pure. The float phases were dramatically different.
Dyrkacz and Bloomquist
378 Energy & Fuels, Vol. 6, No. 4, 1992
APC8-7
0.8
1.1
1.3
1.2
1.4
Density (g m")
Figure 7. S/F separation of the 1.265 g cm-3 CFC float fraction of APCS 7. A 50 g/L sample was centrifuged for one hour at 10000 rpm (16000g)in a swinging bucket rotor. "he vertical line represents the cut density. Table V. Final Purity Data for Separated Density Fractions of APCS 7 density, g cm-3 from to % purity Stage 1 93.8 1.3351 1.335 1.192 96.1 99.2 1.192F
p=1.192F
1.0
1.1
1.2
1.3
5
1.4
Density (g ~ m - ~ )
Figure 8. Final separated fractions from the low-density separation of APCS 7 coal. 100 4
I
! n
Stage 2
1.245 1.24 1.365 1.311 1.265F
1.365 1.311 1.265
91.0 95.1 75.6 66.7 41.1
Whereas at 1.365 and 1.311 g cm-3 there was better than 93% pure material, at 1.265 g cm-3 we have only a 41% float phase purity. This decrease in apparent purity is expected, although the magnitude of the change is larger than predicted. (See Discussion section.) To see whether the purity of the 1.265 g cm-3 float fraction could be improved, we recycled it. However, simple sink/float (S/F) centrifuge separation methods were used, rather than continuous flow separation methods, for four reasons: (1) The amount of material in question was relatively small and could easily be managed within the limitations of small scale S/F separations. (2) Continuous flow centrifugation is a dynamic separation; i.e., particle residence time is critical to separation efficiency. Under our operating conditions, the primary separations were very efficient. Unless we decreased the flow rate considerably, recycling would provide only a small improvement in the purity of the float phase. On the other hand, S/F separations are open-ended in terms of time constraints. (3) We were concurrently examining the nature of simple maceral S / F centrifugation and had developed a fair understanding of the technique and its limitations> (4) We believe that it is important to realize that many different techniques for maceral separation have merit. Any one technique will have limitations that can be overcome or supplemented with another technique. Figure 7 shows the density distributions of the original sample and the final float and sink phases from the S / F separation with one hour centrifugation time. The final float phase purity was 73.8% and the sink phase purity was 86.3% . Thus,the original CFC float phase, with 41 % purity, could be substantially improved through further (0) Dyrkacz, G.R.; Ruacic, L. R.;Fredericke, J. Energy Fuels, to be
published.
80 EO 40
20 108
80 EO
40 20 0 1.1
1.2
1.3
1.4
1.6
Density (g ~ m - ~ )
Figure 9. Final separated fractions from the high-density separation of APCS 7 coal.
separation. The float phase purity could probably have been further improved by even longer S/F centrifugation times. The data in Table I11 represent the purity information at only a single density cut. Table V shows the final purities of all the individual fractions that were isolated. Figures 8 and 9 display the final density distributions for the fractionation. The data in Table V are the final purities of the material within the stated density ranges. Except for the lowest and highest density fractions, each fraction is the product of two CFC separations. The lower purity values in Table V, compared to Table 111, reflect the fact that each separation is not perfect. Separation of PSOC-732. This coal has relatively high proportions of all three maceral groups, which can be seen from both Table I and Figure 2. These proportions make
Energy & Fuels, Vol. 6, No. 4, 1992 379
Coal Maceral Separation. 2
1
Table VI. Percent of Float and Sink Expected from PSOC-732 at Various Densities density, g cm-3 float w t % sink, wt % 1.228 4.9 95.1 1.251 8.4 91.6 1.297 51.1 48.9 1.332 72.2 27.3
1.332
Table VII. PSOC-732 Separation Results in CsCl/Brij-35 float phase
densitycut 1.332 1.297 1.251 1.228
sink phase”
startwt, g
% pure
wt,g
% pure
wt, g
100.0 70.1 52.6 11.6
96.0 89.5 54.5 66.8
70.1 52.6 11.6 7.2
87.6 90.1 96.8 98.3
31.7 13.9 37.9 3.7
”The rotor slurry and the true sink phase were directly combined and treated as sink data.
1 1
1.297F
1.250F
1.1
1.2
1.3
1.4
Denslty (g cm-3)
Figure 11. S/F separation of 1.228 g cmS CFC float fraction of PSOC-732. A 50 g/L sample was centrifuged for one hour at 10000 rpm (16000g) in a swinging bucket rotor.
1
1.220F
Figure 10. Multiple density fractionation of PSOC-732. See Figure 6 for key. Starting coal = 100 g; [Coal] = 75 g/L: flow rate = 25 mL/min; centrifuge speed = -45000 rpm. this coal a good candidate for concentrating each maceral group. To separate this coal, we began with the highest desired density cut and proceeded to lower density cuts. Each successively lower density fiactionation was applied to the preceding float phase. The float material was not isolated in each cycle. A progressive dilution of the coal in the float phase occurred, due to removal of material as sink phase, rotor flushing, and the adjustment of the solution density to the next lower density. In all,four density fractionations were done: two to the high-density side of the main (vitrinite) peak, and two to the low-density side. We started with 100 g of coal at a concentration of 75 g/L in CsCl/Brij-35 solution. The starting volume was 1.33
L;thisvolume evolved to 3.5 L, just before the final density cut at 1.228 g ~ m - ~ . Table VI shows the float and sink proportioning of material that would be predicted, if the deaignated density cuts were separately made on the original coal. From these data, with 100 g of starting coal, there should be no density where we need to worry about overloading the rotor, providing we fractionate the coal from the highest density to the lowest. Figure 10 shows the sink and float phase results of all the CFC separations on this coal, while Table W providea the detailed data. This coal was run as a “blind” series, with no ADGC monitoring of intermediate results. This was done to test the effectivenessof the separation scheme as a routine operation. Thus, none of the intermediate density fractions were recycled. From Table VII, it is apparent that all the sink cuts were quite pure, regardless of whether the density of the cut was less than, or greater than, the density of the vitrinite band (compare with Figure 4). The float fractions, on the other hand, were much more variable. The low-density fractionation at 1.251 g cmV3,which was closest to the main vitrinite peak at 1.278 g showed the worst separation with a purity of only 54% pure float. From Figure 10, this low purity is due to contamination by the large vitrinite band. As with the APCS 7 coal, this result was expected, but the purity is lower than calculated (see Discussion section). To determine if some inherent limit of separation had been reached, the final float phase from the 1.228 g cm-3 separation was recycled using S/F centrifugation techniques. The results are displayed in Figure 11. The float phase was indeed improved, going from the original 66.8
Dyrkacz and Bloomquist
380 Energy & Fuekr, Vol. 6, No. 4, 1992 Table VIJI. Purity Data for Final Density Fractions of PSOC-732 density, g cm+ from to % purity 1.332 > 87.6 1.332 1.297 75.5 1.297 1.251 88.3 1.251 1.228 23.7 1.228 < 66.8
11
12
13
14
15
Density (g ~ m - ~ )
Figure 12. Final separated fractions from the fractionation of
PSOC-732. to 86.2% purity with a centrifugation time of 1 h. The final purity information on the fractions, which blends the results of the CFC separations that bracket or define each fraction, is shown in Table VIII and Figure 12. The data here are similar in character to that observed for the APCS 7 coal sample. The lowest purity fraction was again just to the low-density side of the main band. Figure 12 clearly shows why some float purities are so low. Essentially we are seeing a persistent contamination of the floats by the main vitrinite band. This contamination is due to the naturally higher portion of fines expected in the larger mass of material. This phenomenon was discussed in detail in part 1. Separation of APCS 3. This coal was quite different from the previous two coals that we examined. From Table I1 the amount of liptinite and inertinite available in this coal is much lower than for the previous coals. From strictly the point of view of maximizing the final yield of the liptinite or inertinite group, this coal is not a desirable choice. The isolation of liptinite and inertinite from this coal is a severe test of the efficiencies of any maceral separation. This was one reason for choosing this coal. However, a second reason for working with APCS 3 is that Dyrkacz et al. have shown that the vitrinite group per se has a range of chemi~try.~.~?' Thus, separation of even a high vitrinite coal should be an important consideration in any fundamental studies on coal. Our interest was not only in separating the low concentrationmacerals, but also fractionating the vitrinite. This means that finer density cuts were required than with the previous coals. (7) Dyrkacz, G. R.; Bloomquist, C. A. A.; Ruscic, L. Fuel 1984, 63, 1367-1373.
As expected, this coal was more difficult to separate. However, we found that some of the difficulties had nothing to do with the concentration of maceral groups or the CFC separation technique per se. We have already alluded to most of the operational problems in part 1,but we will elaborate on the problems here, because of the importance for maceral separations in general. One problem we experienced was that CFC-separated fractions of APCS 3 coal, which were further processed by S / F centrifugation techniques, often agglomerated in CsCl/Brij-35 density gradients. This aggregation was related to Centrifugation time for the gradients and also may have been related to the age of the original FEM ground, chemically demineralized, feed coal. Separated phases that were centrifuged in gradients for two or more hours exhibited aggregation; the extent of aggregation appeared to be greater with longer centrifugation times. Fresh coal samples appeared to aggregate more readily than older samples. Moreover, repeated cycles of separation enhanced the aggregation. Yet separated float or sink phases (from S/F separations), suspended in the original separation media for several months, then reanalyzed by ADGC, showed no additionaltendency to aggregate when only one hour of centrifugation time was used. Nor did the samples show changes in density band shape or density position. Unfortunately, we do not know if CFC fractions themselves show this effect. This problem was discovered after we had completed the CFC work described below. A second problem we noted were definite changes in the position of the density distributions when CFC separated fractions were dried prior to density analysis. ADGC comparisons of float or sink phase samples that had been filtered to a wet cake, against samples that had been filtered and dried at 64 "C overnight in vacuo, showed shifta to higher densities in the dried samples. The magnitude of the shift was about +0.005 g cm-3 for a float fraction, and up to +0.01 g cm-3 for a sink fraction. It is not clear to us why there was a difference between the float and sink fractions. Nevertheless, these density shifts caused large changes in the purity information derived from integration of the density distributions. For example, the experimental purity of one float fraction changed from 85.0 to 61.6%, comparing a wet to a dry fraction, respectively. For the corresponding sink fraction, we found an increase in purity from 21.0 to 71.8%, again comparing a wet to a dry fraction. Part of the reason for the dramatic differences between the wet and corresponding dried fractions can be attributed to the very narrow density distribution bands for some of the CFC separated fractions. A small shift in density near the edge of a very narrow peak can dramatically effect the outcome of the purity determination. We do not understand the reason for the differences between the sampling methods. Two possibilities are oxidation of the coal surface or physical changes in the coal structure upon drying. What is clear is that this Illinois No. 6 seam coal is quite different from the other coals. Neither of the other coals exhibited these aggregations or drying problems, even after as many as four S/F recycles. Our response to these problems was to avoid drying the fractions until the ADGC results had been satisfactorily obtained. Also, the time for centrifuging the gradients was strictly maintained at 1h. However, as stated before, we cannot be sure that the underlying cause of the problem was not also affecting the CFC separations. Having addressed some of the obstacles encountered with APCS 3, we can proceed to investigate the finer details of the separation. Figure 13 and Table IX show the data for the separation starting with 250 g of coal. The
Coal Maceral Separation. 2
Energy & Fuels, Vol. 6, No. 4, 1992 381
Table IX. Separation of APCS 3 Illinois No. 6 Coal' density, float sink" ~ c m - startingwt,bg ~ %pure wt,bg %pure wt,bg From High to Low Density 1.352 250.0 93.6 229.4 87.6 23.5 1.310 229.3 93.8 220.5 88.7 5.9 1.296 220.5 90.6 209.1 72.0 7.4 1.285 209.1 93.4 178.4 27.0 27.2 1.264 178.4 42.0 47.3 75.3 118.4 1.264 47.3 49.3 46.3 93.5 1.64 1.250 46.3 56.0 26.0 89.4 19.4 1.220 22.0 52.9 10.1 95.9 16.2 From Low to High Density 15.0 74.5 0.91 12.7 67.4 0.79
1.250 1.263
97.6 91.4
12.7 11.2
L 1.362
1
1.310F
1
1.296F
1
1.285F
1 Figure 13.
-1.0
80 -
- 0.8
60-
-0.6
ECD
h
1.220F
Multiple density fractionation of APCS 3. See Fie;ure 6 for key. Starting coal = 250 g; [Coal] = 71.9 g/L: flow rate = 24-27 mL/min; centrifuge speed = 45 000-48 OOO rpm.
.
< z
-0.4
d
4-
L
0'
s
40 -
2
0
Includes both rotor slurry and sink. *The weights for some of the fractions (such as the starting weights) were determined from assays of the pham. This method is not very accurate for slurries, and, with normal losses during a run, accounts for some of the reasons that the float and sink phases do not s u m to the starting weights. cConditions: starting concentration = 71.9 g/L; flow rate = 24-27 mL/min; centrifuge speed = 45000-48000 rpm.
1
100 -
1.1
o
1.2
i
1.3
:
1.4
I
1.5
-
:
Denslty (g ~ m - ~ )
Figure 14. Comparison of experimental float and sink phase purities for APCS 3. Table X. Percent of Float and Sink Mass Expected from APCS 3 Illinois No. 6 at Various Densities density, g cm-8 float, w t % sink, wt % 1.220 6.6 93.4 1.250 12.3 87.7 1.265 20.1 79.9 1.285 56.6 43.4 1.300 82.6 7.4 1.310 87.5 12.5 1.350 94.8 5.2
rotor slurry generally accounted for 40-90% of the total sink material and typically was less pure than the sink material deposited on the rotor wall. Seven different density cuts were made on this coal, starting from a high There . was also one CFC recycle density of 1.352 g ~ m - ~ at 1.265 g cm4. Each succeeding cut was made on the float phase from the preceding density cut. The starting volume of coal slurry was 3.3 L, but by the conclusion of the 1.285 g cm-3 density separation, the volume had increased to 6.5 L. The volume increase, as stated before, was due to the amount of clear solution used initially to load the rotor, to flush our residual coal, and to adjust the density of the feed to the new density value. It was impractical to procees this volume of solution through the centrifuge in one day. Therefore, the volume was reduced by filtration to a more manageable volume of 3.1 L. This volume of liquid (or actually the remaining amount of Brij-35) was still sufficient to maintain a good coal dispersion. From the data in Table IX,it is apparent that the CFC separations of APCS 3 exhibited dramatic variations in purity. This behavior can be seen more clearly from Figure 14,where we have plotted the float and sink phase purities as a function of the density cut. There is an obvious loss of efficiency near the main band, but the magnitude of the variation is much greater then we expected. The sink density separation is the most phase of the 1.285 g affected. This density is very near the maximum in the ). the data in AF'CS 3 density band (1.284g ~ m - ~ From Table X, we expected that about 26.0% of the original coal should have separated as sink, while only 10.9% was actually recovered. The low purity of the float phase at a density of 1.264 g cm-3 is partially a result of inadvertently overloading the rotor. We found 118 g of coal in the sink phase, which is about twice our recommended amount. This prompted us to recycle this fracton in an attempt to improve ita purity. However, only about 12% improvement in the purity of the resulting float was observed. The float phase from the recycle was then used for the subsequent cut at 1.250 g ~ m - ~ . A second partial separation of an APCS 3 sample was done starting at a density lower than the main band (Table
Dyrkacz and Bloomquist
382 Energy & Fuels, VoE. 6, No. 4, 1992
1
Table XI. Final Fraction Purities for APCS 3 density, g cm-3 from to % purity From High to Low > 90.1 1.352 1.352 1.310 67.3 1.296 34.1 1.310 17.3 1.296 1.285 1.264 52.8 1.285 17.2 1.264 1.250 1.250 1.220 29.5 < 1.220 53.3
APCS-3
,&
1
E
.-cn
i!
From Low to High 1.250 1.250 1.265
74.5 25.0 91.4
IX). This separation consisted of only two cuts, at 1.250 g cm-3 and subsequently at 1.265 g ~ m - Compared ~. to the correspondingcuts in the first series, the increasing density series shows higher purities for both the float and the sink phases. We can compare these low purity values to the single cut data in part 1. This earlier work found 63% pure float phase at 1.250 g cm-3 and 57.8% pure float phase at 1.265 g ~ m - The ~ . values at 1.250 g cm9 for both the low to high fractionation and the singlecycle separation should be the same. Generally, the precision of the ADGC data, when considering any single phase, is around f 5 % . Our precision between CFC runs was expected to be greater. The average purity for the values between the two 1.250 g cm-3 density cuts is 68.6%; thus, the data fall within about &8% of this average. Our impression from other work is that this is probably close to the true precision. However, both the purity data from the single-cycle runs and the low- to high-density fractionation are consistently higher than the high- to low-density data at 1.250 and 1.265 g We expect that, in a multipledensity fractionation sequence, whichever phase is used for succeeding separations (after the first cut) should show an improvement in purity. This should be true because each preceding fractionation eliminates material that is a potential contaminant in the succeeding fractionation. For example, consider a sink phase from a separation at 1.25 g cm-3 that is next separated at 1.265 g ~ m - The ~ . potential amount of contamination by the float phase should be less than ~, we have with a single cycle cut at 1.265 g ~ m - because already removed much of the material x1.250 g cmP3. This consideration should be particularly important when many fractionations are done over small density increments. Thus, we suspect that there may be some "aging" effect that occurs in APCS 3, which can directly affect the separation. This may be related to the problems which we have initially discussed regarding this coal. The overall results of the separations are displayed in Table XI and Figure 15. As expected, some fractions show very poor enrichment. As with the other coals, this is
11
recycle density, % purity 1.265 1.265 1.285
CFC float, % purity 57.8 62.8 77.6
13
14
16
Density (g ~ m - ~ )
Figure 15. Final separated fractions from the fractionation of
APCS 3.
especially true of those fractionations close to the main vitrinite density band. We subjected the CFC separated fractions to further S/F centrifugation separations, to see if the purity of the fractions could be improved. The 1.264-1.285 g cm-3 fraction,which was the sink material generated in the 1.264 g cm-3CFC separation, was recycled at a density of 1.285 g ~ m - The ~ . course of the recycles is displayed in Table XII (last entry). The original CFC fraction was 77.6%pure (float). This was based on a wet analysis; a dried fraction was used for the separation and this indicated only 55.5% pure due to the band shift upon drying. The first S/F recycle, which was done in a large rotor at a relatively slow speed (7000 rpm), showed the float phase improved to 82.8% pure. A second recycle, at much higher centrifuge speeds and for a longer time, brought the final float purity up to 94.0%. There is some caution needed when interpreting this data, because the dried CFC fraction did exhibit a density shift relative to the original density pattern run on a wet sample. In a sense, we were not just cleaning up a fraction, but we were doing a new separation. Nevertheless, the data suggest that there were no inherent limits built into the coal sample that preclude further separations to upgrade the material. The float phases from single cycle CFC separations of the original coal at 1.250 and 1.265 g cm-3 were also recycled; the purity data are also displayed in Table XII. Both of the CFC float fractions were substantially improved by doing S/F recycles, just as with the multifrac-
Table XII. Sink/Float Recycle of 1.265 g
density, g cm-3 1.265F 1.250F 1.265Sc
12
recycle 1 float, % sink, % purity purity 81.2 71.21 95.8 76.d 82.8 76.4d
Single-Cycle CFC" sink/float separations recycle 2 float, % sink, % purity purity 79.1 79.11 93.1 62.g 94.0 20.7e
recycle 3 float, % sink, % purity purity 87.8 66.4'
"All recycles were done on preceding S/F float phase. '50-mL centrifuge tube; 6 h at loo00 rpm.; starting [coal] = 50 g/L. 'This is from the sink 1.265-1.285 g cm-3 fraction from the high to low multiple separation in Table IX. d250-mLcentrifuge bottle, JS 7.5 rotor, 3 h at 7400 rpm (1OOOOg); [coal] = 100 g/L. '5O-mL centrifuge tube; JS-13.5 rotor, 4 h at 1OOOO rpm. (16000g). '50-mL centrifuge tube; 1 h at loo00 rpm.
Coal Maceral Separation. 2 tionated sample. Note that in the density 1.265 g cm-3 case, it was necessary to use long centrifugation times to achieve good disengagements of phases. In Table XII, the sink fractions generally became progressively less pure as we recycled the floats. This result is expected. As we recycle the floats, there is not only less sink material to be separated, but after each recycle the resultant sink phase consists of finer and finer float and sink material. The result is that the sink phase will ultimately become a sharp density band, with its maximum at the density of the solution.6 This is due almost comletely to the float and sink fines. (Similar behavior would be expected for the float phase, if we repeatedly recycled a sink fraction.)
Discussion We believe that our results show that continuous flow centrifugation can separate macerals and produce quite respectable enrichments, even when they are finely ground as with a fluid energy mill. However, there are restrictions on the separation that must be recognized. Some of these will have already been apparent from the data. As we stated in the Introduction, our goal here was not necessarily to provide material that was absolutely pure. We already knew from the previous paper that this would be unlikely with CFC using an FEM ground product. The purpose here was to extend the CFC separation to multifractionation of single coal samples. With the information we have developed from the current separation work and from the work in part 1,we feel we can now point out some of the important elements that must be considered for successful maceral separation by CFC methods. We also recommend examining the references used in this articles, which address points that wiU not be reemphasized or duplicated here. We will discuss some of the issues which commonly emerge in a CFC separation, following the logical order in which they would usually occur during the design of a maceral separation. Some of the points will have an impact on maceral separations in general. It should be obvious that this discussion must be restricted to the typical starting material with which we prefer to work, i.e., a fine ground FEM product that has been demineralized. Preseparation Considerations. For most maceral separations it is prudent to have some idea of the maceral density distribution of the coal in question. Knowing how the macerals are distributed as a function of density will minimize the number of fractionationsthat will be needed to obtain a pure product. It would be of little benefit, but much depression, to arbitrarily choose maceral separation densities which inadvertently enrich the multimaceral particle regions. The most convenient and informative way to obtain the maceral distribution is by using density gradient techniques. However, these techniques require some expertise. In lieu of DGC separations,we recommend that at least small scale S/F centrifugation separations be done to establish the basic density positions where the macerals separate. If CsCl/Brij-35 is used with conditions similar to that described in this work and ref 6, a fair idea of the distribution can be developed. (There is a time penalty for the S f F separation, though.) The need to augment this information with maceral analysis will depend, to a large extent, on how distinct the density divisions of the various macerals are. Dyrkacz and co-workers have shown that there are density overlaps in the maceral The overlaps are caused by two phenomena: natural overlaps of monomaceral particles of different maceral groups, and overlaps due to the presence of multimaceral particles.
Energy & Fuels, Vol. 6, No. 4, 1992 383
150 1
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2
100
It 50
0 1.1
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Density (g c r r s )
Figure 16. Possible fractionation scheme for separation of the three maceral groups from PSOC-732.
Typically the best way to reduce the effect of the latter on the separation has been to fine grind the coal, but even fine grinding is not completely successful. Moreover, when using a dynamic separation method such as CFC, ultrafine particles are the major cause of reduced efficiency. A common alternate S/F separation approach is to use stages of grinding and separation, followed by maceral analyses to monitor the progress of separation. However, this approach can result in large losses of desirable macerals, because maceral liberation is often inefficient at the larger particle sizes (especially for liptinites). Often the final separation must still be done on very finely ground material. In addition, with larger particles, maceral analyses to monitor the density position of the maceral groups is critical. Only rarely will the positions of even the three major maceral groups be apparent. The maceral analyses are also time-consuming. We prefer the initial fine grinding method, because with coals such as APCS 7 and PSOC-732, it is often easy to decide where the three maceral groups are. APCS 3 coal, however, is one case where, even with fine grinding, a maceral analysis is necessary to determine where the maceral groups are. For its excellent wetting and dispersion capacity in maceral separations, we use CsCl/Brij-35 solutions. We disperse a coal sample in this media using mild ultrasonic treatment on batches of prestirred coal slurry. In the present work we have adapted a standard ultrasonic flow cell to disperse the particles in the solution, with one pass through the flow cell. This experimental arrangement is especially convenient in CFC separations, where we are dealing with large volumes of solutions. Although we did not directly connect the ultrasonic flow cell to the continuous flow centrifuge, there should be no problems doing this. Separation Considerations. The next problem that is usually encountered when considering a maceral separation is deciding the number of CFC separations that are necessary to fractionate a single coal. If high-purity fractions of the three major maceral groups are desired, it is imperative to remove the overlapping maceral regions. A minimum of four density cuts is necessary for such a separation. A miniumum of five cuts is needed if there are minerals present. Figure 16 shows an example of the positions of a priori density cuts for PSOC-732. Even after chemical demineralization, minerals, such as pyrite and rutile, will still remain. For separations of the three major maceral groups that are typically found in the literature, the sequence of five cuts would have been sufficient. However, through the use of DGC techniques, there is a large body of evidence accumulating that there are significant physical and chemical variations within maceral
Dyrkacz and Bloomquist
384 Energy & Fuels, Vol. 6,No. 4, 1992
groups.2*"" Thus, five separation cuts should be considered a minimum. The choice of where to start a multidensity fractionation sequence is still not completely clear. In a highly efficient wetting and dispersing media, there is no distinct theoretical advantage where the separation is started. Comparing the single-cycle separation data in part 1with the multiple cut data here suggeata that this is true for at least APCS 7. But APCS 3 coal showed some distinct differences between single cycle runs and high to low (or low to high) density fractionations. The fact that agglomeration problems were noted with this coal may indicate one possible cause for the separation hysteresis. However, from a practical viewpoint there are two considerations to bear in mind when deciding how to fractionate a coal. One is the desire to minimize the volume of liquid which will accrue as the fractionation proceeds; this affecta the time for a separation. The second problem is the ease of manipulations between fractionations. In a high- to low-density fractionation, where successive float fractions are used, the volume of liquid that must be processed at each step gradually increases, due to unavoidable mechanical manipulations. This mode of fractionation has the advantage that overloading the rotor with Sinkmaterial can be minimized. On the other hand, a lowto high-density fractionation minimizes the increases in solution volume due to altering the solution density for each successive separation. However, there is the constant annoyance of having to resuspend the deposited sink fraction to carry out the succeeding separations. In this case, with expensive CsCl that must be recycled, the best course is first to filter the float phase and then use the supernatant for subsequent separations. This sequence involves a lot of manipulation and further presenta the coal with opportunities for oxidation during the filtration and ultrasonic redispersion. A third separation option is to start the fractionation sequence at an intermediate density and work the float phase out to lower densities and the sink phase out to higher densities. This approach reduces the problem of having periodically to stop the rotor to remove the sink phase material. Unfortunately, the fractionation of the sink portion still involves all of the previously discussed manipulations. Our own preference is to work from the highest density cut to the lower density cub. If the volume to be processed becomes too great to conveniently handle, it can be filtered down to a convenient level (solution volume and time) for separation (as we did for the Illinois No. 6 separation). Another critical consideration in this type of separation is the expected purity of maceral fraction as a function of density. In part 1, we developed a straightforward calculation to determine the purity of a float phase, as a function of density under continuous operation of a CFC centrifuge. Ehentially, the particle size distribution is used with the separation dynamics to predict a fractional mass decay curve as a function of density difference between particle and liquid. This decay curve describes the efficiency of separation. When this curve is multiplied by each density point in the density distribution of the coal, we can predict the expected phase purity at any density. Figure 17 is a comparison of the experimental data obtained in the present work with the calculated float phase (8)Taulbee, D.; Pce,S.H.; Robl, T.;Keogh, B. Energy Fuels 1989,3, 662-670. (9) Crelling, J. C. h o c . 1988 Ironmaking Conf. 1988,47, 351-356. (10)Karae, J.; Pugmire, R. J.; Woolfenden, W. R.; Grant, D. M.; Blair, S.Int. J. Coal Geology 1985,5, 315-338. (11) Dyrkacz, G . R.; Bloomquiet, C. A. A.; Ruscic, L.; Crelling, J. C. Energy Fuels 1991,5, 155-163.
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preceding fractionations.
purity curves for all three coals. There is one caveat in this comparison. The calculations are based on the individual cuta to the original coal and not based on material that would have already been separated. The consequence of this is that the displayed theroretical purity values are a lower limit. However, we doubt there would be a dramatic difference, given the particle size distributions that we seem to have. Of course, the first fractionation in each series would not be affected by this fact. As can be seen, only APCS 7 shows reasonable correspondence. The other two coals do not show good quantitative correlation. Given our uncertainty in the particle size distributions and lack of knowledge of the centrifugal dynamics in the rotor, we are not surprised. Nevertheless, although the magnitude is not very accurate, the overall pattern of the experimental pointa does follow what we expect. In Figure 17, the low purities of the experimental sink fractions are more surprising to us. In our original calculations, the only way of obtaining an impure sink fraction was by considering a non-steady-state calculation which took into account insufficient flushing. Barring particlefparticle interactions, the sink fraction should be pure, because the entire liquid phase is really the float phase in CFC separations. The large volumes of solution that were used in some fractionations, relative to the flushing volume, would have come close to the steady-state condition. The low purity of the sink phase, which is primarily due to the rotor slurry phase, indicates that there must be another mechanism for contaminating the sink phase. As we have indicated in part 1, one possibility for the contamination is that there is a quiescent zone in the rotor. In this zone, the axial flow of fluid through the rotor would be very slow. Another possibility is that some part of the liquid phase is rotating far below the nominal rotor speed. Both of these problems w i l l cause the hold-up of a certain amount of float material, which will be ejected as the rotor slurry portion of the sink phase. In principle, distinguishing the relative importance of these two mechanisms for contamination could be done based on particle size variation in the product phases. However, narrow, well-defined particle
Energy & Fuels, Vol. 6, No. 4, 1992 385
Coal Maceral Separation. 2
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Figure 18. Calculated purities as a function of the number of fractions separated for PSOC-732. The theoretical purity information in Figure 17 was used for this calculation. size material is needed for such work. What is important here is that, as predicted, the purity is dependent on the density chosen for the separation and the density distribution. This behavior is universal to any density separation where some particles have insufficient time to reach the proper phase. There are two general conclusions that we can draw from both the calculated and the experimental data: (1) For densities close to large density bands, we will always see a drop in purity. The float phase purity will be most effected to the low-density side of these bands, and the sink phase purity will be most effected to the high density side of these bands. (2) Unless the cut density is very close to the main density band, most of the impurity will be found close to the cut density. The magnitude of the variation with density will depend on just how efficient the separation operation is. There are two more critical elements concerning CFC maceral separations (actually any biphasic density separation) which become apparent from examining the final phase purities of the density fractions: (1) All density fractions, other than the final, lowest density, float fraction and the final,highest density, sink fraction, are the product of two separations which bracket each fraction. The purities of these fractions must be lower than for singlecycle fractionations. (2) Besides this, the number of fractionations made on a coal sample influences the ultimate purity of each fraction. As we increase the number of density cuts to provide narrower density fractions, the mass of material that we expect within each fraction will be less. If the separation efficiency remains roughly the same, then the amount of contamination must increase, as we go to finer and finer density cuts. For example, we first fractionate a coal at a density which gives us a float phase that is, say, contaminated with 10% sink. We then fractionate this float phase at some lower density, which will correspond to 10% of the original float phase mass partitioning as the desired sink fraction. Finally, the actual purity of the resulting desired fraction, bracketed by these two densities, is only 50% (10% contamination mass and 10% desired fraction mass). We have assumed in this example that sink phases are separated with perfect efficiency. Including a certain inefficiency for the sink phase would further reduce the fraction purity. Figure 18 shows an example of how the purity would be expected to vary for PSOC-732, if we take the theoretical float purity information shown in Figure 17 and consider the effect of progressively finer density fractionations on the coal. The data is generated by dividing up the coal density distribution into equal density intervals, starting at the low density. The calculated float phase purity curve is then combined with the mass in each interval to produce
1.1
1.2
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1.6
Denslty (g ~ m - ~ )
Figure 19. Calculated purity data assuming a constant fraction of impurity for the sink phase, and using the purity data for 0.010 g cm" density intervals in Figure 18. the displayed data. The fraction purity data in Figure 18 can only be taken as a maximum limit of purity. The calculation only considers the amount of sink contaminating the float phase, but does not consider the amount of float material contaminating the sink phase. (We do not have an acceptable calculable model for the amount of float that contaminates a sink.) In effect, we knowingly are dismissing half the problem, since each fraction is formed as a sink from one separation cut and a float from another. From the experimental purity data for the sink phases, we can see that the purity would be even less if we did consider the final float contamination. Our actual sink contamination, which is related to the float purity data, also suggests the purities are even lower than indicated by the theoretical calculation. An idea of how including the sink phase impurity would affect the data can be derived by assuming a constant percentage of inefficiency for the sink phase. If we then combine this with the theoretical purity information for the float phase, the results in Figure 19 are obtained. As expected, even a small sink phase separation inefficiency has a profound impact on the final purities of the fractions. At first glance, the preceding discussion is depressing. A worthy and valuable goal would be to fractionate a maceral group into a sufficient number of fractions to be able to reduce and help define the intramaceral variability. (The measure of variability would be defined by subsequent tests.) But conflicting with that goal is the fact that the purity itself is rapidly dwindling, the finer the density resolution becomes. The situation is not quite as unworkable as it may seem, for two reasons. First, as the experimental data show, the preponderance of the impurities will appear at densities lying close to the cut density. Thus,the separated fractions will still have a much narrower density range than the original coal. Second, although a high-resolution CFC density separation series (with a large number of density cuts) cannot provide high-purity fractions unless the separation efficiencies are very high, recycling will improve the fractions. Recycling using CFC alone shows only modest improvement in fraction purity, because for FEM ground materials, CFC separations may be near the limits of convenient operation. S/F centrifugation separation, however, does offer a way out of this dynamic problem. This was one of the underlying reason for performing S/F separations on several of the CFC-separated fractions. Once the initial fractionation has been done by CFC, the resulting fractions can be more easily processed by using S/F centrifugation techniques. This is not to say that S/F is ultimately a better, or more useful, technique than CFC. Each separation method has its own niche. S/F centri-
386 Energy & Fuels, Vol. 6,NO.4, 1992 fugation is strictly a batch technique and cannot conveniently handle large amounts of cod (Le,, large volumes), whereas CFC techniques can be readily scaled up. Alternatively, preparative DGC procedures would be an excellent subsequent method for the processing of CFC separated fractons.8J2 DGC techniques would have a further advantage in not only cleaning up a fraction in one pass, but providing even narrower density fractionation. From our perspective, given the availability of the equipment, this would be the method of choice. The principal lesson to be learned here is that the narrower the density fractionation scheme, the greater the necessity to recycle the density fractions to obtain a well-defined product. For low-resolution separations, such as in the separation of the three maceral groups, we can "trick" the system into a good separation. As an example, we can use the PSOC-732 data in Figure 16. The position of the first lowdensity cut, to separate liptinite from vitrinite, is at 1.223 g ~ m - ~We . know that if we fractionate the coal sample at this density, we will undoubtedly carry over some material more dense than the solution density. From the experimental data near this density, there would be approximately 30% sink impurity in the float phase. Much of this impurity will be vitrinite. Thus, if only a single separation cycle was desired to obtain a pure maceral, it would be prudent to do the separation at some lower density. The actual choice of density depends on the amount of impurity that can be tolerated. Even so, with our current knowledge of CFC dynamics, the desired density is, unfortunately, still in the realm of an experienced guess. The result of starting at the lower density would be an improved single-cycle separation. The cost will be the loss of some desirable product. This method turned out to work quite well for the APCS 7 separation of liptinite from vitrinite. A final word of caution is needed concerning maceral analysis of density-separated materials. The purity (12) Dyrkacz,G. R.; Blwmquist, C. A. A.; Horwitz, E. P. Sep. Sci. Technol. 1981,16,1571-1588.
Dyrkacz and Bloomqukt problems that we have been discussing are mainly due to the presence of very fine coal particles (submicrometer). Traditional optical microscopy techniques for recognition and identification of these fine impurity particles will be impoesible. In effect, these fines will not be counted. This problem can seriously bias the maceral analysin of product fractions by indicating a much higher purity than is warranted. Thus, it is prudent to temper the maceral information on a separated fraction by considering ita density distribution, and the density distribution and maceral information from the original feed coal. If stages of mild grinding and density separation are used instead of directly going to a fine material, this will reduce the identity problem to some extent. However, as we have already said, the later stages of this mode of operation, where fine coal is being separated, will still suffer from the same identification problem. Alternatively, one can consider sieving the coal to remove the fines. This would improve all aspecta of the separation and analysis,but care must be taken to avoid selective removal of some macer&.
Summary We have shown that CFC techniques can be applied to the separation of fiiely ground coal to produce a series of density fractions. Under a specific set of conditions including particle size and centrifuge operation, the purity of the material is controlled by three external factors: (1) the density of the solution used for the fractionation,which in turn is correlated with the density distribution of the coal samples; (2) the separation efficiencies at the highand low-density limits of an individual fraction; (3) the width of the density intervals, or number of fractionations that are generated for a single coal sample. Acknowledlpnent. We gratefully acknowledge that this work was supported under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, under contract W-31-109-ENG38. We also thank Peter G. Vichos (Coulter Scientific Instruments) for the particle size data. Registry No. CaC1, 7647-17-8; Brij-35, 9002-92-0.
